[FIR] Get rid of FirExtension.key

In reality plugin key was used only in two ways:
- found corresponding generator for member scope of generated class
- pass it to declaration origin so it can be read in backend IR plugin

Because of first usage plugin key was required to be an object, which
  reduces functionality of second usage, because there was no way to
  pass any additional info via key to IR (in future this usecase will be
  covered with IR declaration attributes, but right now we don't have
  them)

So now generator which created some class is now saved in its attributes
  and plugins can use any keys they want. And since there is no other
  usages of `FirExtension.key` it removed from FirExtension API at all

plugins/parcelize/parcelize-compiler/src/org/jetbrains/kotlin/parcelize/fir/FirParcelizeCheckersExtension.kt

@@ -10,13 +10,9 @@ import org.jetbrains.kotlin.fir.analysis.checkers.declaration.*
 import org.jetbrains.kotlin.fir.analysis.checkers.expression.ExpressionCheckers
 import org.jetbrains.kotlin.fir.analysis.checkers.expression.FirAnnotationCallChecker
 import org.jetbrains.kotlin.fir.analysis.extensions.FirAdditionalCheckersExtension
-import org.jetbrains.kotlin.fir.declarations.FirPluginKey
 import org.jetbrains.kotlin.parcelize.fir.diagnostics.*
 
 class FirParcelizeCheckersExtension(session: FirSession) : FirAdditionalCheckersExtension(session) {
-    override val key: FirPluginKey
-        get() = FirParcelizePluginKey
-
     override val expressionCheckers: ExpressionCheckers = object : ExpressionCheckers() {
         override val annotationCallCheckers: Set<FirAnnotationCallChecker>
             get() = setOf(FirParcelizeAnnotationChecker)

plugins/parcelize/parcelize-compiler/src/org/jetbrains/kotlin/parcelize/fir/FirParcelizeDeclarationGenerator.kt

@@ -176,7 +176,7 @@ class FirParcelizeDeclarationGenerator(session: FirSession) : FirDeclarationGene
         }
     }
 
-    override val key: FirPluginKey
+    private val key: FirPluginKey
         get() = FirParcelizePluginKey
 
     override fun FirDeclarationPredicateRegistrar.registerPredicates() {